MINOR: trace: also report the trace level in the output

MINOR: trace: also report the trace level in the output

It's very convenient to know the trace level in the output, at least to
grep/grep -v on it. The main usage is to filter/filter out the developer
traces at level DEVEL. For this we now add the numeric level (0 to 4) just
after the source name in the brackets. The output now looks like this :

  [00|h2|4|mux_h2.c:3174] h2_send(): entering : h2c=0x27d75a0 st=2
   -, -, | ------------,  --------,  -------------------> message
    |  | |             |          '-> function name
    |  | |             '-> source file location
    |  | '-> trace level (4=dev)
    |  '-> trace source
    '-> thread number